1. Field of the Invention
The present invention relates to an optical disk having pits, recording marks, grooves, lands and the like for recording information formed partially or entirely on a recording surface, an optical disk reproducing apparatus for reproducing such an optical disk, and to a method of tracking such an optical disk.
2. Description of the Related Art
For an optical disk reproducing apparatus for reproducing information from an optical disk on which information is recorded in advance by pits having recessed and protruded shapes on a disk surface, various proposals have been made as to the tracking servo technique for positioning an optical beam for reproduction on a pit string (track) as disclosed, for example, in Japanese Patent Laying-Open No. 58-1501145.
FIG. 16 is a block diagram of the tracking servo system in accordance with Differential Phase Detection (DPD) method, disclosed in Japanese Patent Laying-Open No. 58-150145.
According to the DPD method, the reflected light beam from the optical disk is received by a photodetector divided into a cross-shape, that is, a photodetector having four areas formed by dividing into two along the radial direction and into two along the tangential direction of the optical disk. A sum signal of outputs of those of the four areas which are positioned at opposing corners is calculated, and a DPD signal indicative of phase difference (time difference) of the sum signals of the opposing areas is detected and used as a servo signal for tracking.
In FIG. 16, the light beam reflected from the optical disk, not shown, is condensed and directed to a photodetector 2. The four areas a, b, c and d respectively output electric signals corresponding to the reflected light quantity. Addition amplifiers 3-1 and 3-2 calculate a sum signal of outputs from areas a and c and a sum signal of outputs from areas b and d, that are positioned at opposing corners of the four areas of photodetector 2, and applies the calculated sum signals to corresponding comparators 5-1 and 5-2, respectively. Comparators 5-1 and 5-2 compare the output signals of addition amplifiers 3-1 and 3-2 with reference signals +Ref1 and +Ref2, respectively, and provide, as outputs, binary signals as a result of comparison.
As the reflected light beam has been diffracted by the pits formed on the optical disk, intensity distribution of the reflected light on the photodetector varies with time, dependent on the positional relation between the optical beam and each pit.
When the optical beam 1 follows just above a pit string, for example, output sum signals (a+c) and (b+d) from the pairs of areas (a, c) and (b, d) at opposing corners of the photodetector positioned above the pits vary in the same manner with time. Therefore, output signals from comparators 5-1 and 5-2 also change in the similar manner at the same timing.
When the optical beam 1 follows positions deviated from just above the pit string, there would be a phase difference (time difference), corresponding to the amount of deviation, between the output sum signals (a+c) and (b+d) of the above described pairs of areas (a, c) and (b, d). Therefore, either one of the sum signals changes first, dependent on the direction of deviation between the optical beam and the pit string.
Therefore, the phase difference (time difference) between the binary signals as the outputs of comparators 5-1 and 5-2 is detected by a phase comparing circuit 7, and a pulse corresponding to the phase difference (time difference) is generated. More specifically, phase comparing circuit 7 compares an R input (output of comparator 5-1) with a V input (output of comparator 5-2), and outputs a pulse of which width corresponds to the phase difference between the two, dependent on which of the inputs is advanced in phase. For example, the pulse of which width corresponds to the amount of delay is output from a U output when the V input lags behind the R input, and output from a D output when the V input is advanced.
The pulse generated in this manner is passed through lowpass filters (LPF) 8-1 and 8-2 to extract only the low frequency components thereof, which are applied to a differential circuit 9. Differential circuit 9 calculates the difference between the low frequency component outputs from lowpass filters 8-1 and 8-2, and supplies the result as a tracking servo signal indicative of the amount and direction of deviation between the optical beam and the pit string (track).
The technique for generating a tracking servo signal other than DPD method described above includes the push-pull method, for example. In the push-pull method, the reflected light beam is divided along the tangential direction of the track, a push-pull signal representing the difference in the quantity of reflected light (difference in intensity distribution) between the inner peripheral side and the outer peripheral side of the disk is calculated, and the signal is used as the tracking servo signal. FIG. 17 is a block diagram of the tracking servo system using the push-pull method.
As already described, when the light beam is directed to a pit string, the reflected light is diffracted by the pit string, dependent on the positional relation between the beam and the pit string. In the push-pull method, the reflected light is divided into two and detected at the inner peripheral side and the outer peripheral side of the optical disk, and a tracking servo signal is generated based on an average light intensity.
Referring to FIG. 17, the reflected light beam 1 is condensed onto a four-split photodetector 2, as in the DPD method shown in FIG. 16. In the push-pull method, however, addition circuits 3-1 and 3-2 add output signals of a pair of areas (a, b) positioned on the inner peripheral side and output signals of a pair of areas (c, d) positioned on the outer peripheral side and outputs the output sum signals (a+b) and (c+d) as the results of addition to a differential circuit 17, not the pair of areas positioned at opposing corners of photodetector 2 as in the DPD method.
Differential circuit 17 calculates the difference between the two sum signals from additional circuit 3-1 and 3-2, and applies the difference as a push-pull signal to a LPF 18. LPF 18 removes high frequency component of each pit from the difference, and extracts the low frequency component, that is, signal component which corresponds to substantial average deviation between the light beam and the pit string, which is supplied as a tracking servo signal. This is the principle of the push-pull method. In the conventional tracking servo control in accordance with the DPD method, push-pull method or the like, polarity of the DPD signal or the push-pull signal may be inverted dependent on the depth of the pit formed on the optical disk, hindering accurate tracking servo control.